1. Area of the Art
The invention relates generally to resistivity measurement of silicon wafers, and more specifically to in-situ epitaxial passivation for improved resistivity measurement.
2. Description of the Prior Art
The net carrier density profiles in epitaxial and polished bulk silicon wafers are often determined by capacitance-voltage (C-V) measurements with a mercury (Hg) probe. The C-V measurement uses the differential capacitance of a depletion layer (whether by Schottky barrier or p-n junction) to determine the majority carrier-concentration profile as the space-charge-region width is changed by a reverse bias voltage.
The method of C-V measurements requires the formation of a Schottky barrier diode with a metal or mercury probe contact to an epitaxial or polished wafer surface. To conduct a capacitance-voltage measurement on an epitaxial silicon wafer having a silicon substrate wafer and an epitaxial layer, a metal or a mercury probe is formed on the surface of the epitaxial layer. When the metal, such as mercury, is brought into contact with a semiconductor, the conduction and valence bands of the semiconductor form a definite relationship. There then exists an energy barrier of a specific thickness--a depletion layer, which represents the energy needed to allow an electron in the metal to escape into the semiconductor. The depletion layer extends from the surface of the metal into the semiconductor, so that a depletion capacitance is formed. The majority carrier-concentration profile in an epitaxial or a polished bulk silicon wafer can then be measured by measuring the differential capacitance of the depletion layer.
The most common problem in capacitance-voltage measurements made with mercury probe instruments is a poor Schottky contact, which is generally indicated by an excessively high leakage current. The use of a poor Schottky contact does not actually prevent a carrier density determination but produces an erroneous result.
It is believed that one of the causes for the formation of a poor Schottky contact is the growth of an oxide layer on the surface of an epitaxial or polished bulk silicon wafer before the formation of the Schottky contact. Thus, in order to produce a reliable Schottky barrier diode, chemical treatment of the silicon surface may be required. A typical treatment of the silicon surface of a wafer requires the use of a chemical treatment with hydrofluoric acid (HF) followed with a deionized (DI) water rinse if the wafer is p-type, or the use of a chemical treatment with hydrofluoric acid followed with hydrogen peroxide treatment if the wafer is n-type.
The HF treatment allows hydrogen to attach to any dangling bonds on the surface of the wafer. This prevents oxygen from bonding, thus preventing growth of an oxide layer. However, such a chemical treatment creates an additional cost to the C-V measurement, and is also time consuming. In addition, the wet chemical process is also a cause for safety concerns.
Therefore, a need exists for providing a silicon wafer having a surface which allows an accurate C-V measurement to be taken without chemical treatments.